Instruments for the detection (including quantitative detection or monitoring) of gaseous chemical species require periodic checking with known reference gas samples to verify their correct operation. Various ways of providing a suitable reference gas sample for calibration purposes are known. These methods include the use of pre-mixed compressed gas standards, calibrated permeation tubes, diffusion tubes, and diluted vapor bubblers.
While some of these alternatives are capable of very high precision and accuracy, known calibration technology tends to be costly, entails inconvenient operations, or requires cumbersome supporting hardware (for example, ovens, pumps, and flowmeters), and is thus impractical for a handheld, low maintenance, or low cost instrument.
A vapor is the volatile, gaseous fraction of a chemical species that exists as a liquid at room temperature and ambient pressure. A very simple and attractive way to generate a vapor is to put the liquid chemical into a partially filled, closed container and allow its headspace to reach equilibrium. At equilibrium, the concentration of the headspace vapor will be dependent on the temperature and pressure of the container and the composition of its contents. The vapor activity of a chemical is defined as the ratio of the partial pressure of the vapor divided by the saturated pressure of the chemical at a given temperature. Therefore, the activity of the vapor at equilibrium with its liquid state in a closed container will have a numerical value equal to one at any given temperature. This means the saturated headspace vapor concentration is a quantitatively reproducible value at a known temperature and pressure.
A difficulty associated with using saturated headspace vapor for the testing and calibration of most chemical detection instruments is that the headspace vapor is usually much too concentrated. In addition, the removal of a large vapor sample from a closed container will result in a severe departures from equilibrium conditions unless the ratio of the volume of the container to the volume of headspace vapor removed is relatively large. Consequently, most instruments require a vapor reservoir of substantial volume to provide a reproducible vapor sample suitable for analysis.
If one could remove a reproducible, small volume from the saturated vapor headspace, then near-equilibrium conditions could be maintained, and a small mass of vapor, appropriate for the sensitivity of the instrument to be calibrated, could be delivered. The problem is to devise a method whereby a small, reproducible volume of vapor can be dispensed from the reservoir containing the saturated headspace vapor.
The most common method involves the use of a microliter syringe. In this manual method, the user inserts a small needle through a rubber septum seal to the vapor container and removes a small aliquot of the headspace vapor. While effective, this method is not well suited for automated operation unless very large and expensive auto-injector robotic systems are employed. See U.S. Pat. No. 5,792,423 (Markelov).
Other methods rely upon the use of stripper gas (also referred to as a “purge” or “carrier” gas), such as nitrogen, to “sweep” a sample of the headspace vapor to a dispersal site, usually a measurement instrument. For instance, see U.S. Pat. No. 5,363,707 (Augenblick et al.), U.S. Pat. No. 6,365,107 (Markelov et al.), U.S. Pat. No. 6,395,560 (Markelov), or U.S. Application Publication No. US 2004/0040841 (Gonzalez-Martin et al.). Another method involves allowing a heated liquid to enter the sample vessel and displace the headspace vapor. See U.S. Pat. No. 6,286,375 (Ward).
Because of the drawbacks attendant upon the above-mentioned approaches, provision of a technology which permits effective testing of a detection instrument while ameliorating such drawbacks would be a significant advance.